The current goal of the aircraft industry is to make products as quiet as possible. One component of noise generation is exhaust noise from an aircraft auxiliary power unit (APU), a gas turbine engine. While the aircraft is on the ground, the exhaust from the APU exits the aircraft and mixes with static air outside. This mixing of high velocity air and static air results in air shear, which causes noise.
This exhaust noise from an APU is generated downstream of any exhaust muffler and thus cannot be attenuated. The only way to reduce the shear effect is to reduce the exhaust velocity.
For a fixed exhaust mass flow, a larger duct results in slower exhaust velocity. A traditional approach to reducing shear has been to reduce the velocity by making the exhaust duct larger. In this case, the larger exhaust duct requires more metal in the liner and muffler. This adds undesirable weight to the system.